1. Field of the Invention
The present invention relates to a low expansion glass useful as photoetching transparent masks to be used for the preparation of semiconductor integrated circuits or as glass substrates for various displays, etc.
2. Description of the Prior Art
Reflecting the increasing trend for higher integration of integrated circuit devices, the glass substrates for photoetching masks are required to be dimensionally stable without undergoing any change with time and to have high heat and chemical resistance as well as excellent thermal shock resistance. On the other hand, from the aspect of the production, such glass substrates are required to be made of a composition which has excellent melting and forming characteristics in order to avoid the formation of bubbles or striae or inclusion of fine foreign matters. As typical examples of the low expansion glass, there may be mentioned quartz glass and pyrex glass. However, these materials have a high viscosity at a high temperature, and it is difficult to melt them. In particular, it is difficult to eliminate the formation of bubbles. For the glass substrates for photoetching masks, even the presence of bubbles or foreign matters as small as a few microns can be a fatal defect. Whereas, soda-lime silica glass containing an alkali metal oxide or an alkaline earth metal oxide has a low viscosity at a high temperature. However, it has a high thermal expansion coefficient and has poor chemical resistance.
A variety of glass plates for photoetching masks have been known. However, it used to be difficult to obtain a glass plate which fully satisfies all the requirements for low expansion characteristics, high chemical resistance and good melting characteristics.